The present invention relates to a plasma processing apparatus for use for producing semiconductor devices and the like by use of low-temperature plasma and a method therefor. In particular, it relates to a plasma processing apparatus suitable for forming uniform-thick insulating film on a surface of a substrate by a CVD method and for performing processing, such as etching, sputtering, ashing, and so on, and a method therefore.
Conventional CVD apparatuses using low-temperature plasma are classified into two types. One type uses a technique in which plasma is generated by applying a voltage of a high frequency within a range of from 10 KHz to 30 MHz to one of parallel plate electrodes in a vacuum (reference is made to Semiconductor Research 18, p.p. 121-170; Semiconductor Research 19, p.p. 225-267). The other type uses a technique in which plasma is generated by leading a microwave of 2.45 GHz into a vacuum chamber. Of those, the technique using parallel plate electrodes has been used mainly.
On the other hand, a problem that characteristics of devices are influenced by a shock of ions in plasma at the time of formation of thin film has become serious with the advance of the fine structure of semiconductor devices.
Increase of the film-forming rate has been required for improvement of processing capacity.
To increase the film-forming rate, it is necessary to increase the plasma density and the concentration of free radicals (active particles just before ionized). To increase the plasma density and the concentration of free radicals, it is necessary to carry out means for increasing energy to be applied and means for increasing a flow rate of reaction gas.
The reason is in that the film-forming rate is apt to be saturated because the quantity of gas decomposed is limited in the case where the flow rate of reaction gas is increased in the condition that energy is not sufficient. Accordingly, it is necessary to supply sufficient energy for decomposition of reaction gas.
In general, the film-forming rate is increased as applied energy, that is, high-frequency electric power, is increased. However, the parallel plate electrodes have a disadvantage in that energy of ions colliding with the substrate increases as the applied energy increases. There arises a problem in that electrical characteristics of semiconductor devices deteriorate.
Further, there arises a problem in that deterioration of gas decomposing efficiency and incorporation of impurities deposited on walls of a reaction chamber into the surface of the substrate are caused by occurrence of abnormal electric discharge.
On the other hand, in the case where plasma is generated by a microwave, the electric field intensity of the microwave is too low to give energy to electrons for ionization if the microwave generated by a magnetron is directly radiated into a low-pressure plasma generating chamber. There arises a problem in that it is difficult to generate plasma.
Therefore, a method in which energy is given to electrons after a microwave frequency is resonated with a cyclotron frequency in which electrons rotate in plasma perpendicular to the magnetic field and a method in which energy is given to electrons after a microwave is amplified to increase the electric field intensity by radiating the microwave into a cavity resonator have been used conventionally.
The former method is called an electron cyclotron resonance (ECR) method. For example, the method has been proposed in Japanese Patent Unexamined Publication No. 56-13480 (U.S. Pat. No. 4,298,419).
The latter method is disclosed in Japanese Patent Unexamined Publication No. 56-96841 and Japanese Patent Unexamined Publication No. 63-103088 (U.S. Pat. No. 4,776,918) proposed by the inventors of the present application.
In the former method, microwave energy is given directly to electrons so that a voltage of a sheath formed between the plasma ad the substrate little changes. Accordingly, both high plasma density and proper ion energy necessary for high-speed processing can be controlled by applying a high-frequency voltage to the electrode mounting the substrate to thereby control the sheath voltage suitably.
However, the ECR method has the following problem, when a high-frequency voltage is applied to the electrode mounting the substrate as described in the Japanese Patent Unexamined Publication No. 56-13480, a high-frequency current is passed between the electrode and a processing chamber provided in the outside of the substrate because there is no ground electrode in the side opposite to the electrode. Accordingly, the effect of ion energy is strong in the peripheral portion of the substrate but weak in the center portion of the substrate. Consequently, the substrate cannot be processed in the uniform condition. Further, the size of the apparatus becomes large because a coil for generating an ECR magnetic field is required.
To solve such a problem and for the purpose of forming film at a high speed by use of proper ion energy, there has been proposed a plasma processing apparatus using a method of radiating a microwave into a cavity resonator through slits as described in the Japanese Patent Unexamined Publication No. 63-103088.
The proposed plasma processing apparatus will be described hereunder in brief.
In general, in the case where a microwave travels in a waveguide or in a cavity resonator which can be considered to be a kind of waveguide, a current corresponding to electric and magnetic fields flows in a surface of the waveguide. When slits are partially provided in the waveguide so as to cross the current, electric charges are accumulated in opposite ends of each of the slits. The accumulated electric charges change with the traveling of the microwave, so that the electric field between the opposite ends of each of the slits changes to thereby radiate the microwave out of the waveguide.
The plasma processing apparatus is based on the aforementioned theory. As shown in FIG. 1, a microwave generated by a magnetron 3 is led into a cavity resonator 1 through a waveguide 2 and then radiated into a plasma generating chamber 6 through slits 4c formed in the under surface of the cavity resonator 1, so that plasma is generated by use of gas supplied through a gas supply pipe 10. A high-frequency voltage is applied to an electrode 7 on which a substrate 8 is mounted. A slit plate 4 arranged so as to be opposite to the electrode is grounded electrically, so that the slit plate 4 is made to be a counter electrode arranged in parallel with the electrode 7. Consequently, the effect of ions can be given to the whole surface of the substrate 8 uniformly.
The aforementioned prior art technique has a problem in that thin film cannot be formed uniformly because there is no consideration on a flow of gas affecting uniformity in film forming.
In the plasma CVD, thin film is formed by means of chemical reaction in the surface of the substrate. Therefore, the flow of gas in the surface of the substrate has a large influence on the thin-film-forming reaction. Thick film is formed in a portion in which a large quantity of gas flows while thin film is formed in a portion in which a small quantity of gas flows. In short, film having a distribution of film thickness is formed on the substrate. The flow of gas changes according to the pressure and the gas flow rate. For example, in the condition that the pressure and the gas flow rate are not more than 10 mTorr and not more than 100 scm.sup.3 /sec, respectively, the effect of diffusion can be obtained mainly to make the concentration of gas molecules in the surface of the substrate so uniform that uniform thin film can be formed.
However, there arises a problem in that the film-forming rate is reduced in this condition becomes the gas flow rate is small.
In most cases, this type plasma CVD apparatus has a heater included in a substrate mount. Because the number of gas molecules between the mount and the substrate is small, the heat transmission rate to the substrate is reduced in low pressure. There arises a problem in that a large time is required for heating the substrate. Therefore, film forming under relatively high pressure has been desired.
As described above, however, in the case where film is formed in a high-pressure region, the influence of the viscosity of the gas flow appears. Consequently, the conventional gas supply method has a problem in that thin film cannot be formed on the substrate uniformly.
Further, the conventional ECR method has the following problem. When a high-frequency voltage is applied to the electrode for mounting the substrate thereon as described in the Japanese Patent Unexamined Publication No. 56-13480, a high-frequency current is passed between the electrode and the processing chamber provided in the outside of the substrate because there is no ground electrode in the side opposite to the electrode. Accordingly, the effect of ion energy is strong in the peripheral portion of the substrate but weak in the center portion of the substrate. Consequently, the substrate cannot be processed in the uniform condition.
In the method using a cavity resonator, on the other hand, the wave length of the microwave changes according to the plasma density when plasma is generated, because the plasma is generated in the resonator. There arises a problem in that the condition of resonance is not satisfied so that the plasma is made unstable. In short, because the condition of resonance is satisfied while plasma is not generated, the electric field intensity of the microwave becomes so large that plasma can be generated. However, when plasma density is increased by generation of plasma, the electric field intensity is reduced because the wave length of the microwave changes so that the condition of resonance is not satisfied. When plasma density is reduced, the condition of resonance is satisfied, so that plasma density is increased. The aforementioned phenomenon makes it difficult to generate plasma stably.
If the electrode to which a high-frequency voltage is applied is provided in the cavity resonator in order to control energy of ions entering the substrate through the unstable plasma, reflection of the microwave occurs. There arises a problem in that plasma becomes more unstable.